+1 for sorting out bike fit, but...

It's worth bearing in mind that the trainer can show up and magnify a lot of small problems. The bike I ride on my trainer is a bit big for me, and I really notice the stretch some days. But, I've ridden the same bike outside for ~15 years, and when I have traffic, scenery, terrain etc, I'm just not aware of the same fit problems. I guess it's because you're moving around more and thinking about it less. It'll still be better to sort out your bike fit, especially on a new bike, but don't lose hope with it or think that it'll never be a comfy bike for you

Also, it may help if you break things up. If you want to spin for a bit, you could increase your cadence, decrease the resistance and sit up for a few minutes: whatever works and gets you enjoying your bike!